This invention pertains to dry, free-flowing, water-soluble salts of chitosan and a method for preparing them.
Chitosan, a deacetylated chitin, is a cation-active poly-primary amine with diverse applications in fields such as dispersing agents, adhesives, pesticides, waste water treatment, food processing and wound healing. Chemically, chitosan is predominantly poly-1,4-(2-amino-2-deoxyglucose). Normally chitosan is prepared from chitin by alkaline hydrolysis whereby the chitin is sufficiently deactylated to become soluble in 1-4% acetic or formic acid. The parent chitin isolate is derived, for example, from crustacean shells by decalcification and deproteinization with acid and alkali.
Rigby U.S. Pat. No(s). 2,040,879 and 2,040,880 (1936) are the classic references to the preparation of chitosan salts and list no fewer than 70 aqueous salt solutions thereof. It was not until the invention of Austin, as disclosed in U.S. Pat. No. 4,574,150, that dry free-flowing salts of chitosan were prepared.
In brief, the process of Austin comprises forming a slurry of chitosan in a liquid reaction medium and thereafter combining the slurry with a defined carboxylic acid. The product salt thus formed was normally recovered by filtration and drying in air. Austin illustrated the liquid reaction medium by disclosure and examples directed to heptane, acetone, 2-butanone, ethyl formate, ethyl acetate, methylene chloride, 1,2-dichloroethane, 1,1,2-trichloroethane, trichloroethylene, perchloroethylene, 1,4-dioxane, acetonitrile, tetrahydrofuran, nitroethane, and 2-nitropropane, optionally containing small amounts of water.
Although the invention of Austin is very useful in that it provides dry free-flowing, water soluble chitosan salts not previously available, the liquid reaction media taught have several disadvantages, especially in commercial use, such as real or suspected carcinogenesis, toxicity, environmental risk, severe flammability, peroxide formation and explosion hazard.
The C.sub.1 -C.sub.3 monohydric alcohols of this invention, taken to mean those alcohols containing one hydroxyl group, are not taught by Austin, it is believed, because such alcohols, absent minimum amounts of water, are either inoperable or react unacceptably slowly under the conditions employed.
Partain et al. in published International Application W087/07618 teach the preparation of several chitosan salts, some of which are comprised by the Austin patent and by the instant invention. Partain et al. teach heterogeneous reaction, as did Austin, between dissolved organic acids and chitosan dispersed in aqueous alcohols, inter alia, containing an amount of water in an amount "up to about 65 wt % of the total medium. . . . , preferably 30 to 45 wt. %, most preferably 40 wt. %."
Partain et al. do not teach that any products were free-flowing.
The Austin patent does not teach the regulation of molecular weight in the product salts; Partain et al. teach against modification of molecular weight.